Air-quench-toughened glass (float glass) plates, particularly such plates having a soda-lime composition, having widely been employed as panes in buildings and as side and rear windows of automobiles. In a conventional method for producing soda-lime glass, in a step for melting glass raw materials at a high temperature in the vicinity of near 1,500° C. in a melting furnace, a nickel (Ni) component contained in stainless steel used for the interior of the melting furnace and Ni-containing metal particles (e.g., stainless steel particles) contained as impurities in a glass raw material may be mingled into molten glass, and the Ni component may react with a sulfur (S) component contained in mirabilite (Na2SO4) serving as a glass raw material. As a result, nickel sulfide (NiS) may be present in a melt-molded glass substrate. The incidence of an NiS impurity in a defective glass product is very low; i.e., the number of impurities is about one in some 10 tons (t) of glass products. In addition, the impurity has a spherical shape and the particle size is as small as 0.3 mm or less, and thus detection of the impurity in a production line is very difficult.
In order to process such substrates formed of soda-lime glass into glass products used in buildings and automobiles, glass substrates are toughened by heating to the-softening point of glass (about 620° C.) and quenching to about 450° C. by means of an air-jet (a so-called quenching step), to thereby generate compressive stress (e.g., 100 kg/cm2 or more) in the surfaces of the resultant glass plates. This process is called air-quench toughening.
When nickel sulfide (NiS) is present as an impurity in air-quench-toughened glass which is heated and cooled to ambient temperature in a toughening step, α-phase NiS, which is stable at about 350° C. or higher, is present in an unstable state. Since (α-phase NiS is unstable at ambient temperature, with passage of time it is transformed into β-phase NiS, which is stable at ambient temperature. The volume of NiS increases concomitant with phase transformation. A toughened glass plate contains a tensile stress layer having a thickness which is about ⅔ the overall thickness of the plate, and thus cracks (breakage) rapidly grow due to an increase in the volume of NiS in the tensile stress layer, to thereby cause spontaneous breakage of the glass plate.
In recent years, these air-quench-toughened glass plates having a large surface area have predominantly served as a building material, and toughened glass products have had a thickness up to 4–19 mm and considerable weight. Thus, as a measure against spontaneous breakage of toughened glass plates induced by nickel sulfide (NiS) impurities or other glass defects contained in toughened glass plates, protective film formed of an organic material is laminated on either side of a toughened glass plate, to thereby prevent the glass plate from becoming a hazard in the event of breakage.
However, a conventional method including lamination with protective film does not serve as an essential measure for preventing spontaneous breakage of toughened glass plates induced by phase transformation of nickel sulfide (NiS).
In addition to the above-described method, there is known a method for removing defective products containing NiS impurities, which method comprises inserting in a firing (soaking) furnace a toughened glass plate which has been heated during a toughening step and annealed to ambient temperature; heating again to a predetermined temperature (typically 290° C. or higher), and maintaining the temperature for a predetermined time to thereby cause phase transformation of NiS from unstable α phase (α phase is stable at about 350° C. or higher) to β phase, which is stable at 290° C. or higher, concomitant with an about 4% expansion in volume; and compulsorily breaking any NiS-containing, defective toughened glass plate through drastic growth of generated cracks. This method is called a batch-manner soaking process.
In such a batch-manner soaking process, a toughened glass plate containing NiS must be broken, to thereby reliably remove any defective product.
A conventional batch-manner soaking process requires a long time and great amount of thermal energy for elevating temperature, since a toughened glass plate which had once been cooled to ambient temperature is heated again to a predetermined temperature. In addition, the time during which the glass plate must be maintained at a predetermined temperature varies with the thickness of the glass plate, to thereby elevate production cost for glass plates.
Furthermore, in a conventional batch-manner soaking process, the phase transformation of nickel sulfide depends considerably upon the temperature elevation rate. Therefore, a variety of conditions must be investigated so as to determine optimum operational conditions.
In addition, in a conventional batch-manner soaking process, when the composition or thickness of a glass plate during temperature elevation is altered, conditions for causing phase transition of nickel sulfide (predominantly maintenance temperature and time) vary. Thus, removal of all toughened glass products containing nickel sulfide is difficult.